Do not report ABSTRACT_MEMBER_NOT_IMPLEMENTED on enum classes

Partial fix of KT-3454

compiler/testData/diagnostics/tests/enum/enumImplementingTrait.kt

@@ -0,0 +1,21 @@
+trait T1 {
+    fun foo()
+}
+
+enum class EnumImplementingTraitWithFun: T1 {
+    E1 {
+        override fun foo() {}
+    }
+    <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>E2<!>
+}
+
+trait T2 {
+    val bar: Int
+}
+
+enum class EnumImplementingTraitWithVal: T2 {
+    E1 {
+        override val bar = 1
+    }
+    <!ABSTRACT_MEMBER_NOT_IMPLEMENTED!>E2<!>
+}